fix(lib): use proper extension

docs/config/index.md

@@ -212,7 +212,7 @@ export default defineConfig(async ({ command, mode }) => {
   {
     "exports": {
       ".": {
-        "import": "./index.esm.js",
+        "import": "./index.esm.mjs",
         "require": "./index.cjs.js"
       }
     }

docs/guide/build.md

@@ -111,7 +111,10 @@ module.exports = defineConfig({
     lib: {
       entry: path.resolve(__dirname, 'lib/main.js'),
       name: 'MyLib',
-      fileName: (format) => `my-lib.${format}.js`
+      fileName: (format) => {
+        const extension = format === 'es' ? 'mjs' : 'js'
+        return `my-lib.${format}.${extension}`
+      }
     },
     rollupOptions: {
       // make sure to externalize deps that shouldn't be bundled
@@ -134,7 +137,7 @@ Running `vite build` with this config uses a Rollup preset that is oriented towa
 ```
 $ vite build
 building for production...
-[write] my-lib.es.js 0.08kb, brotli: 0.07kb
+[write] my-lib.es.mjs 0.08kb, brotli: 0.07kb
 [write] my-lib.umd.js 0.30kb, brotli: 0.16kb
 ```
 
@@ -145,10 +148,10 @@ Recommended `package.json` for your lib:
   "name": "my-lib",
   "files": ["dist"],
   "main": "./dist/my-lib.umd.js",
-  "module": "./dist/my-lib.es.js",
+  "module": "./dist/my-lib.es.mjs",
   "exports": {
     ".": {
-      "import": "./dist/my-lib.es.js",
+      "import": "./dist/my-lib.es.mjs",
       "require": "./dist/my-lib.umd.js"
     }
   }

packages/vite/src/node/build.ts

@@ -681,7 +681,8 @@ export function resolveLibFilename(
       'Name in package.json is required if option "build.lib.fileName" is not provided.'
     )
 
-  return `${name}.${format}.js`
+  const extension = format === 'es' ? 'mjs' : 'js'
+  return `${name}.${format}.${extension}`
 }
 
 function resolveBuildOutputs(